1. Field of the Invention
The present invention relates to a pattern matching apparatus and a scanning electron microscope which extracts a pattern of a semiconductor device from a photographed semiconductor device image and matches CAD data of the semiconductor device with the pattern extracted from the photographed image of the semiconductor device, thereby making it possible to evaluate the pattern and search for an inspection position.
2. Related Art
Recently, minute patterns that are smaller than the wave length of a light source of an exposure device have been processed during a semiconductor manufacturing process by executing the optical proximity correction (OPC) of the reticle (an original pattern for exposure). The effects of such corrections are verified by matching the image of the semiconductor device photographed by a scanning electron microscope (hereafter, referred to as SEM) with CAD data information representing on the pattern.
In measuring a pattern on the semiconductor device, a pattern matching process that uses a template registered beforehand is conducted in order to identify a measuring position on the image photographed with the SEM. Generally, prior to the measuring inspection process, an image photographed with the SEM, the image being photographed of the vicinity of the measuring section on the semiconductor device is registered as the template; during the inspection process, the position that matches the template is searched for from the image of the semiconductor device photographed with the SEM. Then, measurement is conducted at the identified inspection position. However, frequently photographing images for templates during the measuring process results in significantly decreasing the throughput of the entire inspection process. Accordingly, it is required that templates be created by using CAD data, thereby eliminating the prior process and increasing the throughput of the entire inspection process.    [Patent document 1] Japanese Application Patent Laid-Open Publication No. Hei 06-104320    [Patent document 2] Japanese Application Patent Laid-Open Publication No. 2000-293690
A method or device disclosed in patent document 1 (Japanese Application Patent Laid-Open Publication No. Hei 06-104320) and patent document 2 (Japanese Application Patent Laid-Open Publication No. 2000-293690) conducts a pattern matching process by using the above-mentioned CAD data and an image of the semiconductor device photographed with the SEM. However, in the method disclosed in Japanese Application Patent Laid-Open Publication No. Hei 06-104320, a threshold value is provided to isolate SEM-specific noise from the pattern when extracting the pattern from the semiconductor device image photographed with the SEM. However, SEM noise changes according to the condition of the SEM imaging section or the condition of the specimen. Therefore, the threshold value must be frequently changed during the semiconductor inspection process, which may cause efficiency decreased in the inspection process. Also, the setting of the threshold value cannot be intuitively determined from the SEM image, and knowledge and experience of the image processing technique are required. As a result, there is a problem in that it is difficult for a SEM operator to make the settings.
Furthermore, Japanese Application Patent Laid-Open Publication No. 2000-293690 does not provide specific descriptions of the technique for extracting a pattern from the SEM image; the technique is important for conducting the matching process.
A SEM emits an electron beam onto a specimen, collects secondary electrons emitted from the irradiated surface of the specimen, amplifies them, and modulates the brightness thereby forming an image. Due to influences of noise generated by irregular thermal motion of electrons in the specimen or noise generated in peripheral circuits, random noise superimposes on a photographed image. Therefore, a pattern extracting technique that accurately separates noise from the pattern is required.
As stated above, when conducting a pattern matching process for the semiconductor inspection by using CAD data and a semiconductor device image photographed with the SEM, it is important to accurately isolate noise contained in the SEM image from the pattern. Also, in terms of inspection efficiency, image processing parameters used for the pattern matching process should be easily set without needing special knowledge about the image processing technique.